Personally, the most important:
-Possibility to change margins even if they are fixed in the css.
-Having a correct hyphenation rule for French: 3-3.
-Back of the progress bar in the book, with chapter's ticks, in the reading menu.
-Back of the time permanently displayed in the read menu.
-Have a warning of low battery, before it is empty.
-Native support of the tag "series".
-Getting on the cover, not on chapter 1, when you open an eBook.